Numerous products of commercial significance are formed of elastomeric compositions wherein particulate filler is dispersed in any of various synthetic elastomers, natural rubber or elastomer blends. Carbon black (abbreviated herein as CB), for example, is widely used as a reinforcing agent in natural rubber and other elastomers. It is common to produce a masterbatch, that is, a premixture of filler, elastomer and various optional additives, such as extender oil, and then in some cases to blend such masterbatch with additional elastomer in a subsequent mixing step.
Carbon black masterbatch is prepared with different grades of commercially available carbon black which vary both in surface area per unit weight and in structure, which describes the size and complexity of aggregates of carbon black formed by the fusion of primary carbon black particles to one another. Numerous products of commercial significance are formed of such elastomeric compositions of carbon black particulate filler dispersed in natural rubber. Such products include, for example, vehicle tires wherein different elastomeric compositions may be used for the tread portion, sidewalls, wire skim and carcass. Other products include, for instance, engine mount bushings, conveyor belts, windshield wipers and the like.
Good dispersion of carbon black in natural rubber compounds has been recognized for some time as one of the most important objectives for achieving good quality and consistent product performance, and considerable effort has been devoted to the development of procedures for assessing dispersion quality in rubber. The mixing operations have a direct impact on mixing efficiency and on macro-dispersion. In general, better carbon black macro-dispersion is achieved in a dry-mixed masterbatch by longer mixing and by more intensive mixing. Unfortunately, however, achieving better macro-dispersion by longer, more intensive mixing degrades the elastomer into which the carbon black is being dispersed. This is especially problematic in the case of natural rubber, which is highly susceptible to mechanical/thermal degradation. Longer and more intensive mixing, using known mixing techniques and apparatus, such as an internal mixer, reduces the molecular weight of the natural rubber masterbatch-composition. Thus, improved macro-dispersion of carbon black in natural rubber is known to be achieved with a corresponding, generally undesirable reduction in the molecular weight of the rubber.
In addition to dry mixing techniques, it is known to continuously feed latex and a carbon black slurry to an agitated coagulation tank. Such “wet” techniques are often used with synthetic elastomer, such as styrene butadiene rubber (SBR). The coagulation tank contains a coagulant such as salt or an aqueous acid solution typically having a pH of about 2.5 to 4. The latex and carbon black slurry are mixed and coagulated in the coagulation tank into small beads (typically a few millimeters in diameter) referred to as wet crumb. The crumb and acid effluent are separated, typically by means of a vibrating shaker screen or the like. The crumb is then dumped into a second agitated tank where it is washed to achieve a neutral or near neutral pH. Thereafter the crumb is subjected to additional vibrating screen and drying steps and the like. Variations on this method have been suggested for the coagulation of natural and synthetic elastomers, for example, in commonly owned U.S. Pat. No. 4,029,633 to Hagopian and in U.S. Pat. No. 3,048,559 to Heller. Additional wet masterbatch methods are described in, e.g., U.S. Pat. No. 6,841,606, PCT Publication WO 2006/068078, and PCT Publication WO 2006/080852. As used herein, “wet mixing” or “wet masterbatch” techniques refer to methods in which elastomer latex is combined with particulate filler slurry fluid to produce elastomer composite. The resulting elastomer composite is termed a wet mix composite or wet masterbatch. In contrast, dry mix composites are prepared by dry mixing methods in which dry particulate filler is combined with rubber.
An alternative mixing method is disclosed by commonly owned U.S. Pat. Nos. 6,048,923 and 6,929,783, which disclose a wet masterbatch process in which separate streams of a carbon black slurry and an elastomer latex are combined under conditions where the elastomer latex coagulates without the use of added coagulants. The masterbatch is dewatered to about 15% to 25% water content and then passed through a continuous compounder and, optionally, an open mill. An additional method of dewatering and drying a wet masterbatch to optimize the microdispersion of the resulting elastomer composite is described in US Patent Publication No. US 20110021664.
Desirable properties for filled elastomer composites include high wear resistance, high durability, and low hysteresis. Increased reinforcement of elastomer composites, e.g., through higher filler loadings, can improve durability and wear performance. Unfortunately, hysteresis also increases with filler loading. High hysteresis reduces the automobile mileage and is detrimental to elastomer performance in vibration isolation applications. Low hysteresis is also correlated with low heat buildup.
Mastication of dry masterbatch (e.g., after it is produced by a dry mix process or by a wet masterbatch process, followed by drying) may be employed to reduce Mooney viscosity and improve processability while incorporating additives such as oils, antioxidants, and zinc oxide. Vulcanizing agents may be added as well or may be added in a second mastication step. However, the mixing may need to be done at lower temperatures (e.g., below 125° C.) to prevent precure or scorch. In addition, overmixing may be detrimental to viscoelastic properties and may increase flocculation during storage, which can increase storage hardening and further degrade rubber performance (Wang, et al., KGK Kautschuk Gummi Kunststoffe, Vol. 7-8, 2002, pp. 388-396). Thus, it is desirable to have methods for combining vulcanizing agents with elastomer composites produced by a wet masterbatch method that do not compromise the mechanical properties of the resulting vulcanizate.
For some applications, it is desirable to employ blends of elastomers to optimize the mechanical properties of the masterbatch and/or a vulcanized rubber product of the masterbatch. Blends may be produced by co-coagulating a mixture of elastomer lattices (see, e.g., U.S. Pat. No. 4,271,213) or by recovering a polymer blend from a mixture of an elastomer latex and a solution containing a second polymer (see, e.g., U.S. Pat. No. 5,753,742). Alternatively, blends of elastomers may be produced by dry-mixing two elastomers together. It is known to blend dry mixed elastomer composites with additional elastomer to reduce hysteresis. However, the typical reduction thus obtained is typically less than 10% with respect to the original neat material. Thus, it is additionally desirable to produce elastomer composite with even lower hysteresis but without significantly reducing durability and/or wear performance.
U.S. Pat. No. 7,105,595 B2, to Mabry et al., incorporated herein by reference in its entirety, describes elastomer composite blends prepared by wet/dry mixing methods and apparatus. In the wet mixing step, for instance, elastomer composite is prepared by the wet masterbatch method disclosed in U.S. Pat. No. 6,048,923. The coagulum produced by such wet mixing step, with or without intermediate processing steps, is then mixed with additional elastomer in a dry mixing step, for example, during compounding to form elastomer composite blends. The additional elastomer combined with the coagulum may be the same as or different from the elastomer(s) used in the wet mixing step.
However, additional dry mixing of elastomer composites produced by wet masterbatch techniques introduces all the risks to material properties that were originally averted by avoiding dry mixing of the elastomer. Nonetheless, diluting elastomer composites with additional elastomer presents economic and materials benefits to the manufacturer. By preparing more highly loaded elastomer composite (i.e., with greater amounts of filler), additional elastomer can be blended while maintaining a desirable filler loading level. In addition, where the second elastomer material does not have the same composition as that in the elastomer composite, the advantages of the material properties of both elastomers may be exploited. Finally, even where the second elastomer material has the same composition as the elastomer composite, the difference in properties between the filled and the unfilled elastomer can be exploited. Thus, it is desirable to prepare blends of elastomer composites prepared by wet masterbatch methods that can present the advantages above without losing their advantageous mechanical properties as a result of additional mastication during the blending process.